This invention relates to the removal of the sulfate ion from an aqueous sodium chlorate liquor. Specifically, it relates to diluting the liquor with water prior to cooling it to a temperature at which sodium sulfate decahydrate crystallizes but sodium chlorate does not crystallize.
Sodium chlorate can be prepared by the electrolysis of a sodium chloride brine. If the brine contains sulfate ion, the solid sodium chlorate product can also contain sulfate, as sodium sulfate. While for some uses the presence of sodium sulfate in the sodium chlorate is not objectionable, there are other applications where the presence of sulfate is not acceptable.
In U.S. Pat. No. 4,702,805 there is described a method of removing sodium sulfate from a chlorate liquor. In the method described in that patent, a portion of the chlorate liquor is cooled to 5.degree. C. or lower to crystallize the sodium sulfate. However, at that temperature, a significant amount of sodium chlorate also crystallizes. The sodium chlorate that is crystallized with the sodium sulfate can either be discarded and lost, dried and added to the product sodium chlorate, or the cocrystallized products can be redissolved and treated with a soluble calcium or barium salt, such as calcium or barium chloride, to crystallize calcium or barium sulfate. The method of that patent therefore requires both a physical and a chemical crystallization to separate the sulfate from the chlorate.